1. Technical Field
The present invention relates to the field of chemically bonded oxide-phosphate ceramic and, more particularly, to chemically bonded oxide-phosphate ceramic having unique radiation shielding characteristics.
2. Description of the Related Art
Radiation containment, encapsulation, and shielding, including electromagnetic, and microwave shielding, is of increasing and considerable importance in a technologically advanced society. While nuclear power generation offers an alternative to fossil fuel energy sources, containment of waste materials currently raise the expense, thereby decreasing the overall economic feasibility of generating power. Other low level radioactive materials, such as medical wastes, industrial wastes, wastes from depleted uranium ordinance, and the like, also experience the same storage, shielding, and containment issues. Additionally, the proliferation of electronic devices has increased the need to provide effective electromagnetic-shielding. Electronic devices such as cellular telephones, microwave ovens, and the like may require electromagnetic energy shielding that blocks radiated energy from being directed towards the user.
The medical diagnostic field also makes extensive use of radioactive materials to aid in detection of human maladies. The utilization of x-rays and other forms of radioactive material to detect these problems has provided doctors with valuable insight into the patients medical condition. Drawbacks to these diagnostic methods include the shielding necessary to protect the patient and medical personnel from unwanted exposure to radiation and other forms of electromagnetic energy. Currently radioactive medical diagnostics make extensive use of lead as a shielding material. For example, a patient may wear a lead-lined vest to minimize exposure during an x-ray. Lead-lined drywall board is extensively used to provide shielding from primary and secondary x-radiation caused by the primary x-ray beam as well as scattering of the primary x-ray beam during medical x-rays. The x-ray machine itself may require significant shielding, such as provided by lead sheeting, to prevent undue human exposure to radioactive materials.
Metallic lead shielding is extensively utilized because it allows for efficient shielding without unduly consuming space. For example, a sheet of lead less than one inch thick may be implemented to shield an x-ray machine.
Lead shielding drawbacks include the mass of lead, the difficulty in forming structures for holding the lead sheeting in place, the desire for aesthetically pleasing structures, as well as the well-documented carcinogenic human health hazards in the exposure to and handling of lead, and the like. Existing lead-lined bonded gypsum wallboard is very labor intensive to properly install as a secondary and primary x-ray barriers in medical and dental x-ray rooms and facilities.
Other radiation shielding needs include the manufacture of non-lead wallboards that can effectively replace the existing industry standard lead-lined bonded gypsum wallboard used in medical and dental x-ray rooms and similar facilities worldwide. Space stations, satellites, and spacecraft are other areas of possible use for the present invention, as the forms of available radiation shielding materials such as aluminum foil and sheeting, lead dependent materials, and other proposed radiation shielding methods are known to either be minimally effective, require prohibitive thickness contributing to weight problems, sometimes toxic in nature, and often cumbersome relative to the need to develop versatile, strong, durable, relatively easily repaired, composite radiation shielding materials that provide uniquely reliable protective shielding in a space environment.
Utilization of cementious materials to contain and shield radioactive materials, which is described in U.S. Pat. No. 6,565,647, entitled: Cementitious Shotcrete Composition, which is hereby incorporated by reference in its entirety, may be problematic as Portland cement/concrete based systems implement weak hydrogen bonding (in comparison to ionic bonding and covalent bonding). Also these Portland cement based systems suffer from high levels of porosity (in comparison to other matrices, such as a polymeric based material and chemically bonded oxide-phosphate ceramics), corrosion and cracking issues.
Portland cement matrices also require extensive curing (twenty-one days) to ensure proper matrix formation. Other alternatives such as a polymeric based matrix may offer lower porosity but may degrade when exposed to organic solvents and either high or low pH materials. Portland cement matrices also are susceptible to corrosive attack from a variety of materials typically found in radioactive wastes.
Cold-fired ceramic cement materials, such as described in U.S. Pat. No. 5,830,815, entitled: Method of Waste Stabilization via Chemically Bonded Phosphate ceramics, U.S. Pat. No. 6,204,214, entitled: Pumpable/injectable phosphate-bonded ceramics, U.S. Pat. No. 6,518, 212, entitled: Chemically bonded phospho-silicate ceramics, and U.S. Pat. No. 6,787,495, entitled: Multi-purpose Refractory Material, all of which are hereby incorporated by reference in their entirety, do not disclose or suggest incorporating radiopac composite admixtures and therefore do not provide radiation shielding qualities. In an exemplary embodiment of the '815 patent, the following magnesium oxide-phosphoric acid reaction is shown as typical:MgO+H3PO4+H2O→MgHPO4.3H2O
The '815 patent contemplates other metal oxides, including aluminum oxides, iron oxides, and calcium oxides, barium oxides, bismuth oxides, gadolinium oxides, zirconium oxides and tungsten oxides. Minimizing the pH of the reaction, in comparison to a phosphoric acid (i.e., a more basic reaction) is achieved through utilization of a carbonate, bicarbonate, or hydroxide of a monovalent metal reacting with the phosphoric acid prior to reacting with the metal oxide or metal hydroxide. Other contemplated metals (M′) being potassium, sodium, tungsten, and lithium. A partial exemplary reaction described in the '815 patent is:H3PO4+M2CO3+M′Oxide→M′HPO4 
Additionally the utilization of a dihydrogen phosphate to form the ceramic at higher pH (in comparison to the utilization of phosphoric acid) was also indicated in the following reaction:MgO+LiH2PO4+nH2O→MgLiPO4.(n+1)H2O
Fired or low and high temperature curing ceramic materials as described in U.S. Patent Application Publication No. 20060066013 entitled: Low Temperature Process For Making Radiopac Materials Utilizing Industrial/Agricultural Waste As Raw Materials (such as over several hundred degrees Celsius) do not offer a viable alternative to cold-fired oxide-phosphate bonded ceramic structures. High curing temperatures may prevent the materials from being utilized in waste containment and shielding applications as high temperature firing (above several hundred degrees Celsius) requires the components be formed and fired in a remote location prior to transport and assembly in the desired location. High temperature cured ceramics may not be practical for forming large components due to the firing requirements. In-situ formation of fired ceramics for waste containment may be problematic because of the wastes being contained and the location of final storage. Ammonia may be liberated during the firing process. Inclusion of ammonia in the ceramic matrix may be detrimental to the resultant formation.
In U.S. Patent Application Publication 2002/0165082, entitled: Radiation Shielding Phosphate Bonded Ceramics Using Enriched Isotopic Boron Compounds, which is hereby incorporated by reference in its entirety, the utilization of enriched boron compound additives in a liquor solution for phosphate-bonded ceramics so as to provide radiation shielding is described. This document does not suggest radiation-shielding and encapsulation by combining ‘cold-fired’ chemically bonded oxide-phosphate cementitious materials with radiopac fillers and admixtures such as barium sulfate, barium oxide and compounds, gadolinium oxide and compounds, and cerium oxide and cerium compounds, tungsten oxides and compounds, and depleted uranium oxide and compounds.
U.S. Patent Application Publication 20050258405 entitled: Composite Materials and Technologies for Neutron and Gamma Radiation Shielding, which is hereby incorporated by reference in its entirety, describes the use of various radiopac composite material admixtures that are in some applications bonded by various modified Portland cements, grouting materials, epoxies, and magnesium oxychloride/phosphate cement. It is important to note while magnesium oxychloride/phosphate is a similar sounding and written description of a cementitious bonding technique, it is in fact a distinctly different cementitious bonding technique, and one that is known to produce a more porous and less advantageous result to the embodiments disclosed herein below regarding magnesium oxide-monopotassium phosphate cementitious bonding qualities. This published patent application neither includes nor recognizes the potential superior qualities and benefits of chemically bonded oxide-phosphate cementitious techniques for the creation of useful composite material radiation shielding.